Referring to FIG. 1, an exemplary magnetic storage system 10 such as a hard disk drive is shown. A buffer 12 stores data that is associated with the control of the hard disk drive. The buffer 12 may employ SDRAM or other types of low latency memory. A processor 14 performs processing that is related to the operation of the hard disk drive. A hard disk controller (HDC) 16 communicates with the buffer 12, the processor 14, a host 18, a spindle/voice coil motor (VCM) driver 20, and/or a read/write channel circuit 22.
During a write operation, the read/write channel circuit (or read channel circuit) 22 encodes the data to be written onto the storage medium. The read/write channel circuit 22 processes the signal for reliability and may include, for example error correction coding (ECC), run length limited coding (RLL), and the like. During read operations, the read/write channel circuit 22 converts an analog output from the medium to a digital signal. The converted signal is then detected and decoded by known techniques to recover the data written on the hard disk drive.
One or more hard drive platters 24 include a magnetic coating that stores magnetic fields. The platters 24 are rotated by a spindle motor that is schematically shown at 26. Generally the spindle motor 26 rotates the hard drive platter 24 at a fixed speed during the read/write operations. One or more read/write arms 28 move relative to the platters 24 to read and/or write data to/from the hard drive platters 24. The spindle/VCM driver 20 controls the spindle motor 26, which rotates the platter 24. The spindle/VCM driver 20 also generates control signals that position the read/write arm 28, for example using a voice coil actuator, a stepper motor or any other suitable actuator.
A read/write device 30 is located near a distal end of the read/write arm 28. The read/write device 30 includes a write element such as an inductor that generates a magnetic field. The read/write device 30 also includes a read element (such as a magneto-resistive (MR) sensor) that senses the magnetic fields on the platter 24. A preamplifier (preamp) circuit 32 amplifies analog read/write signals. When reading data, the preamp circuit 32 amplifies low level signals from the read element and outputs the amplified signal to the read/write channel circuit 22. While writing data, a write current that flows through the write element of the read/write channel circuit 22 is switched to produce a magnetic field having a positive or negative polarity. The positive or negative polarity is stored by the hard drive platter 24 and is used to represent data.
Referring now to FIG. 2, an input of an amplifier 40 receives analog read signals from a read/write head 42. The amplifier 40 amplifies the read signals and outputs the amplified read signals to a level shifter 44. The level shifter 44 outputs a shifted voltage signal. The level shifter 44 shifts the voltage range at its input by a constant voltage. For example, the level shifter 44 may shift the range of voltages by a value that is equal to a threshold voltage of a transistor. However, other voltage shift magnitudes are possible. The level shifter 44 outputs the shifted voltage signal to a read channel.
Referring now to FIG. 3, the level shifter 44 includes a source follower module 52, a load module 54, and a bias generation module 56. An MR sensor 58 in the read/write head 42 outputs read signals to an operational amplifier (opamp) 60 in the amplifier 40. The opamp 60 outputs amplified read signals to the source follower module 52 in the level shifter 44. The source follower module 52 communicates with the load module 54 and outputs a first voltage value to the load module 54 based on the amplified signals. The bias generation module 56 also communicates with the load module 54 and generates a bias current for the level shifter 44. The load module 54 receives the bias current and outputs an output voltage value to the read channel. For example, the output voltage value may be equal to a value of the amplified read signals combined with a voltage drop across a resistor in the load module and a threshold voltage of a transistor in the source follower module 52.
The gain of the level shifter 44 may be adjusted by changing the bias current that is output by the bias generation module 56 and/or the value of a resistor in the load module 54. However, the operating parameters of the level shifter 44 are typically set during manufacturing. Therefore, a new level shifter 44 is typically required when an operating parameter of a circuit changes. A new level shifter 44 may need to be manufactured according to a desired specification and/or to suit a particular application. This may be both costly and time consuming when a desired circuit configuration changes.